1. Field of Technology
The present invention relates to a switching power supply device, and relates more particularly to an apparatus that enables power conservation during standby mode operation and high efficiency during operation at a rated load.
2. Description of Related Art
FIG. 9, FIG. 10, and FIG. 11 show examples of a step-down voltage type switching power supply. Step-down voltage switching power supplies include power supply devices with a bootstrap circuit to assure control power for the high side block as shown in FIG. 9 and FIG. 10, and power supply devices that do not have a bootstrap circuit and control the switching power using only the high side block as shown in FIG. 11.
FIG. 9 shows a first example of the prior art described in Japanese Unexamined Patent Appl. Pub. 2000-350440. Operation of the control circuit 108 in the first example of the prior art shown in FIG. 9 is referenced to the ground terminal 106, and derives control power for the high side switching device 107 when the switching device 107 is ON from both ends of the capacitor 110 in the bootstrap circuit 111, to which current is supplied from input power supply 101 through diode 109 when the high side switching device 107 is OFF.
FIG. 10 shows a second example of the prior art described in Japanese Unexamined Patent Appl. Pub. 2001-112241. The power supply device shown in FIG. 10 is a synchronous rectifier type power supply device having a bootstrap circuit similarly to the first prior art example shown in FIG. 9. The main difference between the device shown in FIG. 10 and the first example shown in FIG. 9 is that the diode 103 shown in FIG. 9 is replaced by a switching device 202. A level shifter 209 is also provided so that switching device 201 and switching device 202 are not on at the same time, and the switching devices are controlled so that when one is on the other is off. By using a switching device instead of a diode on the low side, the power supply efficiency is improved by lowering the voltage drop occurring at the ends of diode 103 in FIG. 9 when the high side switching device is off to the voltage drop caused by the on resistance of the switching device 202.
The simultaneous rectifier, step-down voltage type switching power supply shown in FIG. 10 has the following two main features.
(1) Generally when current flow to the high side switching device 107 rises in the prior art example shown in FIG. 9, current flow to the diode 103 also rises when the high-side switching device 107 is off, the forward voltage therefore also rises, and power loss from the diode 103 also rises. Power loss is therefore reduced by connecting a low on-resistance switching device parallel to the diode 103 (or a switching device 202 is used instead of the diode as shown in FIG. 10) so that the voltage produced when the circuit is energized is lower than the forward voltage of the diode 103.
(2) The switching devices connected to the high and low sides are switched on and off by PWM control so that both switching devices are not on at the same time.
FIG. 11 shows a third example of the prior art as taught in Japanese Unexamined Patent Appl. Pub. H10-191625. In FIG. 11 VOUT is the output node voltage, IOUT is the output node current, VDS is the voltage between the drain and source of switching device 302, IDS is the drain current flowing to the switching device 302, and VCC is the voltage at the CONTROL node in FIG. 11. The device described in FIG. 11 comprises an input capacitor 301, switching device 302, a control circuit 303 for the switching device 302, a capacitor 304 for the control circuit reference voltage, a conversion circuit 305, output voltage detection circuit 309, and protection device 310.
When the input terminal voltage VIN (a DC voltage or the voltage from a commercial AC power source rectified by a diode bridge or other rectifier and smoothed by input capacitor 301) is applied to the drain of switching device 302, the internal circuit current supply circuit 311 of the control circuit 303 supplies current through switch 312 to the capacitor 304 connected to the control node, VCC thus rises, and the control circuit 303 starts on/off control of the switching device 302. On/off switching of the switching device 302 is controlled by the comparator 316 comparing the sawtooth wave output signal from the internal oscillator 313 with the voltage-divided VCC output by the two resistances 314 and 315.
Once on/off control of the switching device 302 begins, power is supplied to the conversion circuit 305 comprising diode 306, coil 307, and output capacitor 308, and VOUT rises. VOUT is detected by output voltage detection circuit 309. When VOUT rises to or above a predetermined level, current flows from the OUT node to the CONTROL node of the control circuit 303 when switching device 302 is OFF. As a result, as a result of VCC rising and the ON duty of the output signal from comparator 316 decreasing, the ON duty of the switching device 302 is also short, and the switching device 302 is controlled with PWM control.
PWM control thus seeks to stabilize the output voltage and conserve energy by gradually reducing the ON duty ratio (ultimately lowering the peak of current IDS flow to the switching device) of the switching device as the output load decreases.
FIG. 12 shows a fourth example of the prior art as taught in Japanese Unexamined Patent Appl. Pub. 2003-189632 corresponding to United States Patent Appl. Pub. US 2003/0112040 A1. More particularly, FIG. 12 shows a bridge circuit using HVIC (high voltage driver IC) circuits 450, 451, and 452 for inverter control of a motor, and more particularly shows a three-phase motor drive circuit having three half-bridge circuits parallel connected with the output terminals connected to a motor. Power switching devices 417, 418, 419, 420, 421, 422 in a three-phase (U, V, W) bridge circuit are connected between the high and low potential sides of a main DC power source for inverter drive 423, and diodes 431, 432, 433, 434, 435, 436 are parallel connected to the power switching devices.
HVIC circuit 450 is a single-chip circuit device comprising input signal processing circuit 402, power device drive/protection circuit 412, and a level shifter 437 having a photocoupler and electrical isolation function. A similarly arranged single-chip HVIC circuit 450, 451, 452 is rendered separately for each phase, U, V, and W, but devices having a separate HVIC circuit for phases U, V, and W rendered on a single chip are also known.
The reference potential nodes of the three HVIC circuits and the emitter of each low potential power switching device are connected to the U, V, W phases. The emitters of the high potential side power switching devices and the second reference potential nodes of the high potential side drive circuits connected to the level shifters of the HVIC circuits are respectively connected to the U, V, W phases. The output drive signal nodes of the HVIC circuits are connected to the gate of each power switching device.
The input signal processing circuit of the HVIC circuit is connected to the output port of the microcomputer or other device that generates the control signals for driving the power switching devices, and power for controlling and driving the HVIC circuits is supplied from external power source 430. The power for driving the high potential power switching devices of the HVIC circuit is supplied from a bootstrap power circuit comprising external power source 430, high voltage diodes 440, 441, 442 and capacitors 443, 444, 445 connected in series to the external power source 430 for each U, V, W phase, and the ends of the capacitors 443, 444, 445 connected to both sides of the drive circuits for the high potential power switching devices.
When inverter drive is used with an actual motor, control signals are passed to the U, V, W phase HVIC circuits from the inverter drive control signal generating circuit of the microcomputer, and the power switching devices on the high and low potential sides of the U, V, W phase bridge circuit are switched according to the drive signal to supply AC power between the output nodes and control the motor.
A bootstrap power supply circuit drives the high side power switching circuits in this bridge drive circuit. This bootstrap power supply circuit operates so that when the main DC power source is applied to the bridge circuit, the microcomputer drive signal that drives the low-side power switching device is passed to the HVIC circuit, and the low-side power switching device turns on. Because current flows in this state from the external power source to the high voltage diode, to the capacitor, to the low-potential power switching device, and to the reference node of the external power source, both sides of the capacitor are charged by voltage Vcap as defined in equation (1).Vcap=V(external power source voltage)−Vf−Vc(V)  (1)where Vf is the forward voltage drop of the high voltage diode, and Vc is the collector potential of the low potential power switching device.
Operation of the drive circuit that drives the high side power switching devices of the HVIC circuit is maintained by the power accumulated in the capacitor. Therefore, when the main DC power source is applied to the inverter drive circuit, a charge is not accumulated in the capacitors 443, 444, 445, and the high-potential side drive circuit is therefore unable to operate.
After the main DC power source is applied, a drive signal causing the low-potential power switching device for each phase to stay on for a predetermined time is passed from the microcomputer to the HVIC circuit in order to charge capacitors 443, 444, 445. The motor is then controlled by passing the motor drive control signal from the microcomputer to each HVIC circuit.
If the voltage at both sides of the capacitors 443, 444, 445 is not regularly recharged, the charge stored in the capacitors will drop below the level required to the drive the power switching devices after a certain amount of time due to natural dissipation of the stored charge. A signal causing the low-potential power switching devices to turn on is applied to the HVIC circuits within a maximum time determined by the constant of the inverter drive circuit during motor drive, thereby controlling motor operation with a control signal that causes the capacitors 443, 444, 445 to charge.
The following three problems are present with the simultaneous rectifier method shown in FIG. 10.
(1) The input supply voltage is typically approximately 20 V. This is because signal transmission is required between the high side switching device control circuit unit and the low side switching device control circuit unit in order to achieve PWM control.
(2) If the input supply voltage is greater than 20 V, a bootstrap circuit (bootstrap circuit 111 in FIG. 9 and bootstrap circuit 203 in FIG. 10) for supplying power to the high side switching device control circuit, and a level shifting circuit (level shifter 209 in FIG. 10) for signal transmission are needed. Supplying power to the high-side switching device control circuit unit through the bootstrap circuit and signal transmission by the level shifting circuit from the low side to the high side are limited to periods when the high side switching device is off (that is, when the low-side switching device is on). As a result, on-time control of the high-side switching device is extremely difficult when the high-side switching device is on because the supply voltage of the high-side switching device control circuit discharges naturally and gradually drops. More specifically, this renders operation of the high-side switching device control circuit unstable. This is also the case with (3) described next.
(3) When used with an even high input supply voltage of 60 V or more, for example, the bootstrap circuit for supplying power to the high-side switching device control circuit comprises a diode 440 and capacitor 443, and is connected to a power source 430 that is separate from the main power source 423 connected to the high potential node of the high-side switching device 417. Two or more input power sources are thus required as shown in the inverter drive bridge circuit for a motor in the fourth example of the prior art shown in FIG. 12.
Reducing the size and improving the power supply efficiency of the switching power supply device, and further reducing the power consumption during standby states, and particularly in a no-load state, therefore cannot be expected using a step-down voltage switching power supply according to the prior art.
(1) The peak level of current flowing to the switching device drops in the no-load state with PWM control of the step-down voltage switching power supply taught in the third example of the prior art above, but further reduction in power consumption is difficult because the number of switching operations is constant irrespective of the load.
(2) The bootstrap circuit must be externally attached to the voltage step-down switching power supply described in the first and second examples of the prior art, and this prevents reducing the size of the power supply device.
(3) With the voltage step-down power supply device described in the first and second examples of the prior art, the high-side switching devices are switched on/off by the voltage at both ends of the capacitors in the bootstrap circuit, and a loss of control precision caused by the drop in capacitor voltage and fluctuation in the drain current caused by the fluctuating gate voltage of the switching device both occur easily.
(4) The input voltage range of the voltage step-down power supply device described in the first and second examples of the prior art is limited because use only at a relatively low voltage is difficult.
(5) The low potential side of the voltage step-down power supply device described in the third example of the prior art is composed of diodes as described in the first prior art example. This increases power loss in the diodes during steady state operation, and thus prevents further improvement in power efficiency.
(6) Separate power supplies are required for the control circuit and switching devices when using a high voltage input supply as described in the fourth prior art example.